Southwest’s participation basically guarantees the fare hike will stick because it carries the most domestic passengers. The latest increases are modest — $6 to $10 per round-trip — and they apply only to tickets bought at the last minute. Advance-purchase fares remain unchanged.

Still, the fare increase is another sign that despite an overall slowdown in the economy, demand for air travel remains strong. If the economy continues to slide, though, it’s not clear how much longer carriers will be able to keep raising prices.